The existing techniques propose 2D image, the breakthrough is to propose a 3D system. Currently in use 2D system suffers from a lack of both spatial resolution and material discrimination capability. The projective nature of the images captured by those systems requires that the customs analysts must proceed in a human-mind 3D reconstruction which can be difficult or even impossible in case of complex loading. X-ray cargo inspection has not changed much, in principle, since the start, in the 90s. The inherent limits of planar radiography only allow incremental evolutions and the improvements which it may bring, will remain reduced even if useful for the users.
These limits of radiography mean that today, customs organizations and other authorities need additional technologies in separate set-ups to fully inspect containers. They require solutions that combine effective threat detection with efficient interplay with operational constraints. That is why they inquire for alternatives to X-rays, capable of providing more precise information, for example, on the content of hermetically sealed containers.
Thus, MULTISCAN 3D consortium ambitions to achieve a real technological breakthrough in the NII scanning systems both in terms of application allowed by the used of laser-plasma source for tomography 3D, then in each technological block in terms of progresses beyond the state of the art from an R&D point of view.
Challenge 1: use laser plasma interaction for electron acceleration and X-ray emission
Single/dual view high-energy X-ray transmission imaging
Laser-plasma-based radiation able to produce several other types of radiations. The acceleration and production of electrons X-rays, Gamma rays and neutrons opens the possibility of providing customs with an all-in-one first- and second-line system.
The MULTISCAN 3D project aims at exploiting the opportunities provided by the laser-plasma acceleration technology. The principle of this emerging technology is illustrated in Figure below. A high peak power laser pulse is focused in a gas jet. The rising edge of the laser ionizes the gas and thus creates a plasma. As it propagates in this plasma, the laser pulse expels electrons from its trajectory and generates a plasma wave which is associated with electric fields of up to 100’s of GeV/m. Electrons trapped in this plasma wave can hence be accelerated to relativistic energies on extremely short distances. Each laser pulse produces one electron bunch. X-rays are then produced by the interaction of the electron bunch with a high-Z target, through the Bremsstrahlung mechanism.
Challenge 2: Increase laser average power and laser transport study
Low-speed inspection and rotating set-ups and high radiation dose requirements
Overcome X-Ray single view limits Avoid distortion effect Avoid clutter effect on images
For the use of a laser-plasma X-ray source for cargo-screening application, three main technical issues must be managed:
- Control of laser-plasma interaction physics to reach the required electron and X-ray characteristics,
- Develop laser building blocks for the next generation of femtosecond and high average power laser systems,
- Design an efficient beam transport of femtosecond, high peak power laser beams.
Challenge 3: Assess and develop a 3-dimensional reconstruction for multi-view configuration
Single lateral view limits target definition to shape contour in two dimensions.
From 2D display of the radioscopic images, to a set of conventional 2D X-ray image enriched by 3D reconstructed images.
With this new design, the number of the input projections (10-40 projections) will be greatly reduced compared to usual computed tomography set-ups based on hundreds of projections acquired all around the object with a dense angular sampling.
In the targeted application, only few views will be recorded on a limited angle around the container and analytic reconstruction algorithms will produce strong artefacts in the reconstruction results.
The development of specific algorithms able to produce high quality 3D images from a severely reduced data set will be a major challenge to address.
In this project, we propose to address the sparse-view CT reconstruction task in two steps allowing first to reduce the creation of artefacts in the 3D image, then to restore structures. These steps are realized by applying deep learning approaches first in the projection or ‘sinogram’ domain, then in the reconstructed image domain.
Challenge 4: Ensure the detection of X-rays with multiple sources, dosimetry and beam monitoring
The proposed detector development consists in segmenting the detectors not only in the direction of localization but also in another direction of the irradiation plane. This technology exhibits several technical difficulties and the work will be focused on the following points:
- For one dimension of localization, we need detectors segmented in two directions, hence 2D detectors.
- The cells will not probably have the same dimension in the Y direction. It means that even with the conventional technology (photodiodes/photodiodes) a complete redesign of the components must be done.
- The number of electronic channels will be considerably larger than for usual system and a redesign of the usual electronic will be necessary.
Dosimetry and beam monitoring: An innovative spectrometer will be investigated during the project that consists in a filter-detector stack (FDS) made of metallic filters and OSL/FO detectors assembled in series along the beam direction. OSL/FO are passive dosimeters that provide online dose monitoring with the help of optical fibres. Pulse-by-pulse accumulation may be performed with OSL/FO detectors, thus enabling to improve uncertainty in dose measurement
Diamond beam monitor: A novel pixelated transparent scCVD diamond beam monitor system will be developed for pulse-by-pulse beam position stability and intensity measurements. The beam monitoring of ultrashort X-ray pulses created by laser-plasma interaction is a changing task, due to the femtosecond duration of the pulses and thus extremely high peak intensity of the generated X-ray/gamma pulses. Diamond material due to the large bandgap, extremely high radiation hardness to X-ray/gammas, and fast signal generation is a perfect choice for the fabrication of beam monitoring system.
Challenge 5: Ensure the second line identification
The laser-plasma source can be used to build a new type of pulsed neutron source to achieve cargo container inspection:
- Elemental identification in view of explosive detection by fast neutron transmission spectroscopy,
- Fast and thermal neutron interrogation to detect Special Nuclear Materials by neutron-induced fission particles.
The photofission reaction can be used to detect Special Nuclear Material such as actinides like uranium and plutonium isotopes potentially hidden in a cargo container.
In the frame of this new project, the challenge is to use a laser-driven Inverse Compton Scattering (ICS) beam as high-energy photon source to perform the photofission technique. Such a source would enable to combine different measurement techniques. Indeed, this project will also focus on working on the Nuclear Resonance Fluorescence (NRF) technique which enables to detect light nuclei material including carbon, oxygen, fluorine, etc. Both the photofission and the NRF techniques could be used as second line technologies to inspect suspicious areas in cargo containers.
Challenge 6: Ensure the second line identification
The objective of this task is to carry out a demonstration and experiments with the developed system, in order to evaluate and demonstrate the capabilities of a system that may be built upon this project in a field environment. The main work will be performing the demonstration and experiment themselves, using a mock-up defined and realized during the project